Neurosurgical stimulator



March l5, 1955 2,704,064

J. A. FIZZELL ET AL NEUROSURGICAL STIMULATOR Filed Sept. l0, 1952 E VI..

United States Patent() NEUROSURGICAL STIMULATOR James A. Fizzell andJames G. Golseth, Pasadena, and Robert W. Kay, Altadena, Calif.,assignors to The Meditron Company, a corporation of CaliforniaApplication September 10, 1952, Serial No. 308,868

6 Claims. (Cl. 12S-2.1)

Our invention relates to neouromuscular stimulators used by surgeonswhen operating on areas of the body wherein nervous and muscular tissuemay be exposed for direct electrical stimulation.

In many fields of surgery, it has been found advantageous to useelectrical Stimulators to apply electrical current to body tissues, todetermine which of them are electrically excitable tissues.Illustrations of such elds are traumatic surgery, tumor surgery, plasticsurgery, neurosurgery, and orthopedic surgery.

The practical use of direct electrical stimulation to the nerves is wellillustrated by certain tumor removal operations. Tumors often form inthe parotid gland and the tumorous material surrounds branches of theseventh nerve (cranial). This seventh nerve is the nerve which controlsthe facial muscles. When a surgeon dissects this tumorous material, itis almost impossible to visually distinguish between the tumoroustissues and the nerve tissues which pass therethrough. It has been foundthat the only practical way to distinguish these tissues is by usi-ng anelectrical stimulator. The surgeon applies an electrical current to anarea of the tumor and notes whether there is a reaction of a facialmuscle. If such a reaction occurs, the electrode is touching thetumorous tissue in the close vicinity of a nerve, or actually contactingthe nerve. If such a facial muscle reaction does not take place, theelectrode is then touching tumorous material which may be excised. Thus,by careful probing, the surgeon may excise tumorous material withoutsevering any of the branches of the seventh nerve. The severance ofthese branches of the seventh nerve would cause a facial paralysis andhence must be avoided.

Another eld of surgery which illustrates the desirability of directlystimulating exposed nerves is the eld wherein damaged nerves are exposedfor corrective surgery. Pressure may be exerted on the nerves by theformation of scar tissue thereabout, or from any other source, such as abone bearing against a nerve. Another common condition is that the nervemay have a neuroma in continuity (certain cells in the nerve multiplyrapidly and cause an abnormal growth on the nerve). These conditionsgreatly impair the function of the nerve.

One of the most common of these conditions today is that wherein theformation of scar tissue about a nerve impairs its function. A greatnumber of men received injuries on the battlefield during World War IIfor which neither time nor equipment permitted all of the attention thatmight have been needed. These injuries in many instances were permittedto heal so that scar tissue formed about the nerves and subsequentlycreated pressure thereon with a consequent loss of complete muscularcontrol over muscles innervated by the aifected nerves. Surgery must beused to relieve this condition.

` The surgeon exposes the nerve and scar tissue a-nd can then proceed tocheck the continuity of the nerve by stimulating it proximally to thescar tissue. The results of these tests enable the surgeon judiciouslyto remove the damaging scar tissue. Such tests, of course, also wouldindicate whether the nerves are dead, and also probing of the scartissue with the electrodes will enable the surgeon to excise the scartissue without severing a nerve. 1 -The conditions mentioned above whichcreate pressure upon a nerve are quite different from those met intraumatic surgery. In traumatic injuries, areas of the body may be tornopen and nerves and tendons severed. The surgeon must know which tissuesare nerves and which are tendons in order to properly suture them. Since2,704,064 Patented Mar. 15, 1955 ICS tendons and nerves look alike, theonly sure way to differentiate is to stimulate the tissue and notewhether a muscular contraction follows. If a nerve is stimulated, amuscular contraction will follow while the stimulation of a tendon willcreate no such muscular response.

In a neuromuscular stimulator both alternating and direct currents areuseful. The alternating current produces a sustained or tetaniccontraction of a muscle when it ows either through that muscle orthrough its intact nerve supply or through its area of representation inthe cerebral cortex. For comparison, the direct current, when used atlow values, produces only a slight twitch of the muscle at the time whenthe circuit is closed. Some surgeons nd that direct current is moreefficacious in mapping out epileptogenic scars in the cerebral cortex;

The electrical stimulation of a nerve is due to the current passingtherethrough. Hence, a stimulator for use by surgeons must be such thatthe operator knows whether current is being fed to a nerve and theexistence of a circuit fault must be readily apparent in order toprevent misinterpretation.

The body contains various types of electrically conductive uids, and thesurgeon must know when these conductive fluids, rather than the nerve,are passing the current. The oridinary procedure is to raise the nervefrom the vicinity of these conducting fluids and apply the electrodes,but the possibility of conduction through the iluids may neverthelessexist and thereby cause a short circuit path between the electrodes.

Nerves like all other conductors have a current capacity, and if thiscurrent capacity is exceeded, the nerve may be burnt and therebypermanently injured. In testing nerves through the skin, the likelihoodof such burning is greatly reduced, but when the nerves are directlystimulated, care must be taken to keep the current value below thecapacity of the nerve. The surgeon must, therefore, be fully aware ofwhat current is passing between the electrodes in order to prevent theinadvertent burning of a nerve.

Experimentation in stimulating nerves with alternating current hasrevealed that a square wave form of current is the most efficient typeof alternating current. The reason for this is immaterial to the instantdisclosure, but it is significant that our stimulator is adapted to meetthis requirement and provides an approximate square wave of currentbetween the probes.

Accordingly, an object of our invention is to provide a new and improvedneuromuscular stimulator adapted to enable either direct orpercutaneous-stimulation of nervous and muscular tissues.

Another object is to provide a neuromuscular stimu lator wherein bothsquare wave alternating current and direct current are available at theprobes.

Another object is to provide a neuromuscular stimulator which is simpleto operate, apprises the operator of the exact condition under which thecircuit is operating and removes the danger of causing nerve injury.

Other objects are to provide a neuromuscular stimulator which isportable, which is operable from its own power supply, and which meetsall of the peculiar requirements created due to the nature of itsintended use.

These and other objects of our invention will become apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings, in which:

Fig. 1 is a perspective view of a portable neuromuscular stimulatorshowing particularly the controls and probes; and c Fig. 2 is aschematic diagram lllustrating the circuit of the neuromuscularstimulator.

A preferred embodiment of our portable neuromuscular stimulator isillustrated in the drawings, and as seen in Fig. 1, the apparatus ishoused in an open front casing, generally designated 10, provided with ahandle 12. All portions of the neuromuscular stimulator circuit thatmust be accessible and visible for proper operation are mounted on afront panel 14, which is secured to the casing by screws 16. A chassis(not shown) is secured to the rear of panel 14 and adapted to slide intoand out ofthe casing 10. This provides ready accessibility for servicingand replacing the components of the stimulators electrical circuit.

arcanes In the schematic diagram of the stimulator shown in Fig. 2, thesource of power consists of two batteries 18 and 20 connected inparallel. These batteries may, f or example, be ll/z-volt dry cells.Connected in series with the power supply is a fuse 22. The fuse 22,which may have a current capacity of 3 amps., is enclosed 1n a casing 24(Fig. 1) and may be readily replaced by removing casing 24 from frontpanel 14. Also in series with the power supply is a switch 26 which isoperative to turn the electrical stimulator circuit on or off. It willbe noted in Fig. l that the control switch 26 is mounted for convenientoperation on the front panel nA lead 28 connects control switch 26 withthe center tap of primary coil 30 of a step-up transformer, generallydesignated 32. Lead 34 connects one end tap of the primary coil 30 witha contact 36, while another lead 38 connects the other end tap of theprimary coil 30 with contact 40. A wire 42 makes an electricalconnection between the right-hand side of fuse 22 and a vibrator leaf44. An actuating coil 46 is disposed beneath the vibrator leaf 44 andconnected between the center tap of primary coil 30 and a contact 48disposed above vibrator leaf 44.

The vibrator leaf 44 is mechanically xed at its lefthand end asindicated at 50, but is free to move upwardly or downwardly over itsright-hand portion. The leaf 44, however, is normally urged in an upwarddirection either by its own resilience or by a mechanical biasing meanssuch as spring 51.

From the above description it will be seen that at the instant switch 26is closed current will ow from batteries 18 and 20 through fuse 22, lead42, leaf 44, contact 48, actuating coil 46, and through switch 26 backto the batteries. At the same time current will flow from vibrator leaf44 through contact 40, lead 38, the lower half of primary coil 30, andswitch 26 back to the batteries. A magnetic field will be created aroundactuating coil 46 due to current flow therethrough, which magnetic fielddraws vibrator leaf 44 downwardly to break the electrical connectionsbetween leaf 44 and contacts 40 and 48. This movement of leaf 44 causesit to touch contact 36. At this instant current will then be caused toow from leaf 44 through contact 36, lead 34, the upper half of primarycoil 30, lead 28 and switch 26 back to the battery. When the leaf 44 wasremoved from engagement with contact 40, current ceased to ow throughlead 38 and the lower half of primary coil 30. Electrical connectionbetween leaf 44 and contact 48 was also broken when the leaf 44 waspulled downwardly, hence, current no longer flows through actuating coil46 and the forces holding the leaf in its downward position cease. Thiscauses the leaf to spring back to the position shown in Fig. 2 and intoengagement with contacts 40 and 48.

The circuit is then in condition to initiate another cycle such as thatdescribed above. It will be noted that current was first caused to flowupwardly through the lower half of primary coil 30 and then current wascaused to flow downwardly through the upper half of primary coil 30.This reversal of current flow causes the magnetic field of thetransformer to build up first in one direotion, decrease to zero, andthen build up in the other direction. The center tapped secondary coil52 of transformer 32 is linked with this changing magnetic eld and,therefore, alternating voltage is induced in the secondary coil. Thefrequency of this voltage will be mainly clpendent upon the speed ofmovement of vibrator leaf A voltage dropping resistor 54 which may, forexample, have a resistance of 1/2 megohm, and a neon lamp S6 areconnected in series across the outside taps of secondary coil 52. Thus,whenever a voltage is induced in the secondary coil 52, the lamp 56 willglow. In Fig. 1, it will be seen that the neon glow lamp 56 is exposedon the face of panel 14 and, therefore, readily visible -to an operatorof the neuromuscular stimulator to indicate when the stimulator is incondition for use.

A capacitor 58 is also connected across the outside taps of secondarycoil 52. This capacitor may, for example, be of .0l microfarad with apeak voltage rating of 1600 volts. Capactior 58 is used -to tune thesecondary transformer circuit so that the time constant of the circuitagrees with that of vibrator leaf 44. Therefore, it should beappreciated that the capacity of capacitor 58 will depend upon theinductance values of the transformer 32.

The lower tap of secondary coil 52 is connected through a lead 62 to acontact 60, which is disposed above leaf 44, while the upper tap ofsecondary 52 is connected by a lead 66 to a contact 64, which isdisposed below leaf 44. The center tap of secondary coil 52 is connectedto one side of a single-pole, single-throw, switch 68 by a lead 70. Theupper tap of secondary coil 52 is connected to one terminal of asingle-pole, doublethrow switch 72 by leads 66 and 74, while the lowertap of secondary coil 52 is connected to one terminal of a single-pole,double-throw switch 76 through leads 62 and 78.

Single-pole, double-throw switches 72 and 76 and switch 68 are gangedswitches. A voltage dropping element 80 is connected between the switchlegs of singlepole, double-throw switches 72 and 76. A lead 82 isconnected between one side of element 80 and an ammeter 84, which may bea universal meter having a scale range of from 0 to 10 milliampereseither alternating or direct current, and a sliding contact 86 isarranged to bear against element 80. Sliding contact 86 is connected toan electrode terminal 88 by a lead 90, while the ammeter 84 is connectedto a second electrode terminal 92 by a lead 94.

In Fig. 1 it will be noted that ammeter 84 and electrode terminals 88and 92 are readily visible and accessible from the front panel 14. Knob96, in Fig. 1, controls the position of contact 86 along voltagedropping element 80, while lever 98 is the master control for the gangedswitches 68, 72 and 76.

When lever 98 of Fig. l is thrown to its uppermost or A.C. outputposition, switch-arm 76 contacts the terminal connected to lead 78 andat the same time switch-arm 72 contacts the terminal connected to lead74, thereby connecting the voltage dropping element 80 across the endtaps of secondary coil 52.

Leads 97 and 99 are connected to terminals 88 and 92, respectively, andlead into an insulated probe handle 100. Within the handle 100, leads 97and 99 are connected to probes 102 and 104. From this construction itwill be noted that any current passing between probes 102 and 104 willbe indicated by ammeter 84.

When, as previously explained, the magnetic eld surrounding primary coil30 is changing, voltages will be induced in the secondary coil 52. Sincethe tuning capacitor 58 is of a value so as to tune the secondarytransformer coil circuit, a substantially resistive load will bereflected to the primary. Due to the adjustment of the time constant ofthe transformer circuit and to the quick action of the making andbreaking of contact between the vibrator leaf 44 and contacts 36, 40 and48, alternating voltage having a substantially square wave form will beproduced in the secondary transformer circuit. This illustrates one ofthe novel features of our invention since it has been found byexperiment that a square wave form of current is the most efficient typeof an electrical stimulus.

The lamp 56 will be lit when voltage is induced in the secondary coiland thereby indicate that the electrical stimulator is properlyfunctioning. When lever 98 is in the AJ-C. output position, the voltagedropping element 80 is connected to the end taps of secondary coil 52through leads 78, 62, 74 and 66, and switch-arms 72 and 76 and thesquare wave current will flow through element 80.

With the circuit in condition for A.C. output, one of the probes 102 istouched to an exposed nerve while the other probe 104 is touched to thebody in the vicinity of the exposed nerve. The knob 96 may then berotated to move the contact 86 along element 80 to control the amount ofcurrent flowing between the probes. Hence, it will be seen that a squarewave form of current will be applied between the body and a nerve byprobes 102 and 104 with the magnitude of this current being indicated byammeter 84.

The above description has mainly dealt with the portions of the circuitwhich are utilized when an alternating current is to be used as thestimulating current. When direct current is to be used as the stimulus,the switcharms 68, 72 and 76 are thrown into their uppermost position,by moving lever 98 of Fig. 1 to its lowermost or D.C. position.Switch-arm 68 is connected through a lead 106 to an iron core choke 108and a lter capacitor 110. A second filter capacitor 112 is connected tothe other end of choke 108. The filter capacitors 110 and 112 areconnected together by a lead 114. Thus choke 108 and capacitors 110 and112 are connected into a pi filter circuit. The side of choke 108connected to capacitor 112 is also connected by a lead 116 to a terminaladjacent switch-arm 76 while the point of common connection betweencapacitors 110 and 112 is connected by a lead 118 to a terminal adjacentswitch-arm 72. As previously mentioned, contact 60 is connected to thelower end of coil 52, contact 64 is connected to the upper end of coil52, and the center tap of coil 52 is connected to a terminal adjacentswitch-arm 68. With the switches in their D.C. output positions, it willbe seen that the center tap of coil 52 is connected to one input side ofthe pi filter circuit through lead 70, switcharm 68, and lead 106, whilethe other input side of the pi filter circuit, namely, lead 114, isconnected to the vibrator leaf 44 through leads 120 and 42.

When the vibrator leaf 44 is in its uppermost position, it will be notedthat lead 114, of the pi filter circuit, is connected to the lower endtap of secondary coil 52 through lead 62, contact 60, leaf 44, lead 42and lead 120, while, when the leaf 44 is in its lower position, the lead114 will be connected to the upper end tap of coil 52 through lead 120,lead 42, leaf 44, contact 64, and lead 6 The center tap of transformercoil 52 remains connected to the other side of the pi filter circuit atall times that switch-arm 68 is in its upper position, regardless of theposition of the vibrator leaf 44. It should be apparent that the leaf 44and contacts 60 and 64 act as a synchronous rectifier and causepulsating direct current to be applied to the input of the pi filtercircuit. The filtering action of choke 108 and capacitors 110 and 112smooth out this pulsating direct current and thus non-pulsating directcurrent will appear between leads 116 and 118. Since switch-arms 72 and76 are in their uppermost position, this non-pulsating direct currentwill also appear across voltage dropping element 80. Thus, with one ofthe probes contacting the skin immediately above a subcutaneous nerveand the other probe contacting the skin at some other portion, thepresence or responsiveness of the nerve may be tested by passing directcurrent between the probes, the value of which may be adjusted by movingcontact 86 along element 80 and read by ammeter 84.

As previously mentioned, the surgeon using our stimulator during anoperation must know the actual condition of the stimulator circuit inorder to prevent a misinterpretation of the results. As a practicalmatter, the most serious situation is where, upon application of theprobes to a nerve, a deliection of ammeter S6 occurs and a muscularresponse does not follow. The surgeon must be sure that the fault liesin the nerve itself and is not due to a fault of the stimulator circuit,shortcircuiting effects of body fiuids, and the like.

One of the novel features of our neuromuscular stimulator is that thecircuit is so arranged that by a minimum of quick tests the operator mayassure himself as to the reliability of his findings. A glance at glowlamp 56 will indicate whether the circuit is in condition for operation.By touching together probes 102 and 104 and noting that the meter 84deilects, the operator is assured that the circuit is in condition topass stimulating current between probes A102 and 104.

The test to determine whether body fluids or some other conductivesubstance is shorting the probes 102 and 104 is even simpler. If themilliammeter 84 indicates the flow of current when the probes are nottouching anything, then there is some unwanted conductive path betweenthe probes or elsewhere in the electrode circuit. Of course, such afaulty set of electrodes should not be used until the conductive pathhas been cleared. In most cases in the operating room, this will be donewith a piece of gauze or a sponge which will wipe away the blood orsaline that is lodged between the probes. If necessary, however, theelectrodes can be completely rebuilt in very short time or can becompletely replaced with another set in a few seconds.

Having now described our neuromuscular stimulator in full detail, itshould be apparent that the novel device meets all of the variousrequirements necessary in a stimulator for use by surgeons whenstimulating nervous and muscular tissue either directly orpercutaneously. is to be understood that our invention is not limited tothe details shown and described, but may assume various forms,modifications, and equivalents coming within the scope of the appendedclaims. Y

We claim:

1. In a neuromuscular stimulator having probes arranged for stimulationof nerves, a direct current power supply, a center-tapped primarytransformer coil, actuating means including a vibrator leaf connected toalternately pass direct current from said power supply in oppositedirections through first one-half of said primary coil and then throughthe other half of said primary coil, a center-tapped secondarytransformer coil, a filter circuit, a voltage dropping element, anammeter connected in series between said element and probes, a movablecontact engaging said element and in series with said probes, rectifyingcontacts connected to the ends of said secondary coil and arranged toalternately contact said leaf, said leaf being connected to said filtercircuit, and switch means arranged in one position to connect saidelement across the ends of said secondary coil to provide an alternatingvoltage at said probes and in another position to connect said filtercircuit to the center-tap of said secondary coil and to said element toprovide unidirectional voltage at said probes.

2. In a neuromuscular stimulator having probes arranged for stimulationof either nervous or muscular tissue, a direct current power supply, acenter-tapped primary transformer coil, actuating means including avibrator leaf connected to alternately pass direct current from saidpower supply in opposite directions first through one-half of saidprimary coil and then through the other half of said primary coil, acenter-tapped secondary transformer coil, a capacitor connected acrossthe ends of said secondary coil to adjust the time constant of thetransformer, a lter circuit, a voltage dropping element, a currentmeasuring device connected in series between said element and probes, amovable contact engaging said element and in series with said probes,rectifying contacts connected to the ends of said secondary coil andarranged to alternately contact said leaf, said leaf being connected tosaid filter circuit, and switch means arranged in one position toconnect said element across the ends of said secondary coil to providealternating current at said probes and in another position to connectsaid filter circuit to the center-tap of said secondary coil and to saidelement to provide direct current at said probes.

3. In a neuromuscular stimulator having probes arranged for thestimulation of nerves, a direct current power supply, a primarytransformer coil having primary center and end taps, said primary centertap being connected to one side of said power supply, a vibrator leafconnected to the other side of said power supply, contact means adjacentsaid vibrator leaf and connected to said primary end taps, a vibratorleaf actuating coil, said actuating coil being operatively associatedwith said vibrator leaf and intermittently moving said vibrator leaf ina given direction to cause current to flow through said primary coilbetween said primary center tap and one of said primary end taps, saidvibrator leaf being movable in the opposite direction to cause saidcurrent to ow intermittently through said primary coil between saidprimary center tap and the other of said primary end taps, a secondarytransformer coil having secondary center and end taps, a filter circuit,synchronous rectifier contacts adjacent said leaf and connected to saidsecondary end taps, a voltage dropping element, a current measuringdevice connected to said element in series with said probes, a movablecontact connected to said element in series with said probes, and switchmeans arranged in one position to connect said element across saidsecondary end taps to provide alternating voltage between said probesand arranged in another position to connect said filter circuit and saidsynchronous contacts between said secondary coil and said element toprovide unidirectional voltage between said probes.

4. In a neuromuscular stimulator having probes adapted to provideelectrical stimulus to nerves and muscles, a power supply, a vibratorleaf connected to one side of said power supply, a primary transformercoil having primary center and end taps, said primary center tap beingconnected to the other side of said power supply, a rst contact mountedon one side of said leaf and connected to one of said primary end taps,a second contact mounted on the other side of said leaf and connected tothe other 7 of said primary end taps, a third contact mounted on saidone side of said leaf, an actuating coil mounted on' said other side ofsaid leaf and connected between said third contact and said primarycenter tap, said actuating coil being constructed and arranged to movesaid leaf out of contact with said first and third contacts and intocontact with said second contact in response to a magnetic field createdabout the actuating coil by current flow therethrough, means biasingsaid leaf into contact with said third contact when current flow ceaseswithin said actuating coil due to the separation of said third contactand said leaf, a secondary transformer coil having end taps, a condenserconnected between said secondary end taps to tune said secondary coil, avoltage dropping element connected across said secondary coil, a currentmeasuring device connected between said probes and said element, and amovable contact adapted to engage said element and connected in seriesbetween said probes and said element, whereby, alternating current of anearly square wave form will ow between the probes when the latter haveconducting tissue therebetween, which current ow will be indicated bysaid current measuring device and the magnitude varied by moving saidmovable Contact relative to said voltage dropping element.

5. In a neuromuscular stimulator having probes adapted to provideelectrical stimulus to nerves, a power supply, a vibrator leaf connectedto one side of said power supply, a primary transformer coil havingprimary center and end taps, said primary center tap being connected tothe other side of said power supply, a first contact mounted on one sideof said leaf and connected to one of said primary end taps, a secondcontact mounted on the other side of said leaf and connected to theother of said primary end taps, a third contact mounted on said one sideof said leaf, an actuating coil mounted on said other side of said leafand connected between said third contact and said primary center tap,said actuating coil being constructed and arranged to move said leaf outof contact with said first and third contacts and into contact with saidfirst and third contacts and into contact with said second contact inresponse to a magnetic field created about the actuating coil by currentflow therethrough, means biasing said leaf into contact with said thirdcontact when current ow ceases within said actuating coil due to theseparation of said third contact and said leaf, a secondary transformercoil having secondary center and end taps, a filter circuit having inputand output sides. said input side being connected to said secondarycenter tap, a first rectifying contact connected to one of saidsecondary end taps and disposed on said one side of said vibrator leaf,a second rectifying contact connected to the other of said secondary endtaps and disposed on said other side of said vibrator leaf, a leadconnecting said vibrator leaf to said input side of said filter circuit,a voltage dropping element connected across said output side of saidfilter circuit, an ammeter connected in series between said probes andsaid element, and a movable contact engaging said element and connectedin series between said probes and element, whereby said rectifyingcontacts will alternately engage said vibrator leaf to apply pulsatingdirect current to said input side of said filter circuit, whichpulsating direct current will be smoothed out and applied across saidelement, and between said probes said ammeter reading the direct currentow between said probes, which direct current ow may be varied inamplitude by moving said moving contact.

6. In a neuromuscular stimulator having probes adapted to provideelectrical stimulus to nerves and muscles, a power supply, a vibratorleaf connected to one side of said power supply, a primary transformercoil having primary center and end taps, said primary center tap beingconnected to the other side of said power supply, a first contactmounted on one side of said leaf and connected to one of said primaryend taps, a second contact mounted on the other side of said leaf andconnected to the other of said primary end taps, a third contact mountedon said one side of said leaf, an actuating coil mounted on said otherside of said leaf and connected between said third contact and saidprimary center tap, said actuating coil being constructed and arrangedto move said leaf out of contact with said first and third contacts andinto contact with said second contact in response to a magnetic fieldcreated about the actuating coil by current flow therethrough, meansbiasing said leaf into contact with said third contact when current owceases within said actuating coil due to the separation of said thirdcontact and said leaf, a secondary transformer coil having secondarycenter and end taps, a tuning condenser connected between said secondaryend taps to tune said secondary transformer coil and thereby adjust thetime constant of the transformer to agree with that of the vibrator, afilter circuit having input and output leads, a first rectifying contactconnected to one of said secondary end taps and disposed on said oneside of said vibrator leaf, a second rectifying contact connected to theother of said secondary end taps and disposed on said other side of saidvibrator leaf, a lead connecting said leaf to one of said input leads ofsaid filter circuit, a first switch having a first terminal connected tosaid secondary center-tap and a switch leg attached to the other inputlead of said filter circuit, a voltage dropping element, a currentmeasuring device connected to said element in series with said probes, amovable contact adapted to engage said element and arranged in serieswith said probes, a second switch having a first terminal connected toone of said output leads of'said lter circuit and a second terminalconnected to said one secondary end tap and a switch leg connected tosaid element, a third switch having a first terminal connected to theother of said output leads of said filter circuit and a second terminalconnected to said other secondary end tap and a switch leg connected tosaid element, and means for simultaneously moving said switch legs tocontact said first terminals associated therewith to cause directcurrent to flow between said probes and for simultaneously moving saidswitch legs to contact said second terminals associated therewith tocause alternating current to ow between said probes, said currentmeasuring device indicating the current flow between said probes and theamplitude of the current flow between the probes being controlled bymoving said movable contact relative to said element.

References Cited in the file of this patent UNITED STATES PATENTS2,564,279 Reynolds Aug. 14, 1951

